The present invention relates to low-power integrated circuits, and particularly to integrated circuits which can provide data communication over a serial channel.
Battery-powered and portable electronic modules and system have found an increasing variety of applications. In general, such a system or module can provide advantages which may include portability; improved immunity to extraneous electrical noise; persistent memory; improved safety; lighter weight; improved capability for international marketing; and simplified regulatory requirements. Thus, more and more functions have been added into battery-powered systems, particularly for systems which have very low current requirements (and can therefore use small, long-lifetime batteries, such as lithium batteries).
However, one function which is not easy to achieve in such a low-power system is serial data communications. The conventional protocols for serial data communications, if implemented in a straightforward fashion, could rapidly exhaust a battery.
For example, the RS232 standard is very widely used, in microcomputer and minicomputer (and other) systems. RS232 has a number of advantages: it is widely used; the connections are simple (requiring only RX, TX, and ground); and (as actually interpreted by users) the standards are understood to be somewhat flexible, so that adaptation to changing technology is readily possible.
However, the RS232 standard specifies a 3 to 7K ohm load resistor to ground, and normally the RS232 data line will remain in the negative (mark) level when the line is idle. Since this level is below ground voltage, a significant current normally flows in the idle state. (In a battery-powered module, the positive and negative supply voltages could be taken from separate batteries, or a charge-pumping circuit could be used to obtain two supply voltage polarities from a single battery; but in either case the current requirements will still reduce the battery lifetime.) Thus, a substantial current must be sourced whenever a negative level occurs on the incoming line. If a user were to initiate a data communication session, and then leave the interface active when interrupted, this current requirement could rapidly deplete a battery.
The disclosed innovations provide a solution to this problem. The battery-powered module of the presently preferred embodiment steals current from one of the data lines to power the other data line. When the battery-powered module is transmitting a high level, battery current will be used; but, since the low level is (in practice) the default state, high levels occur relatively infrequently, and power consumption due to the high levels will be small.
Among the innovations disclosed in the present application is an integrated circuit which uses an unusual comparator design, which is not fully complementary but which has an extended common mode range, to control switching of one pole of the output driver (e.g. the lower pole) between an internal voltage (e.g. ground voltage) and a current extracted from an incoming signal.